A new report from the American Lung Association (ALA) suggests the U.S. is facing a pollution problem masked in false readings that air quality has steadily improved.

According to ABC News, the "State of the Air Report" found large cities like Los Angeles, New York and Chicago to be most at risk. The ALA said climate change has brought on a rise in ozone levels from 2010 to 2012, most likely causing a decline in air quality.

"We're making progress but some of that could be reversed with rising ozone levels," Janice Nolen, ALA vice president of national policy, told ABC News. "A changing climate is going to make it harder to protect human health."

The report found tiny air pollutant particles to be the culprit and warned that these can be especially dangerous because some air quality measurements can miss them. Air pollution is known to be harmful to seniors and children as well as worsen conditions like heart disease, lung cancer and asthma attacks.

The five communities with the worst air pollution are all in Calif., with the Los Angeles - Long Beach area coming in first.

"Air pollution is not just a nuisance or the haze we see on the horizon; it's literally putting our health in danger," Bonnie Holmes-Gen, ALA senior policy director, told the Los Angeles Times. "We've come a long way, but the status quo is not acceptable."

What is so concerning about the findings is they come at a time when many cities are appearing to show air quality improvement. The report found that 18 of the 25 most polluted cities showed lower particle levels the year before. However, the ALA estimated 140 million Americans are subject to poor air quality.

"Some cities are in a 'bowl' which collects pollutants," Dr. Norman Edelman, a preventive and internal medicine professor at Stony Brook University, told CBS News. "Over the years, extensive epidemiologic studies have shown as much as a 20% increase in ER and hospital admissions of patients with heart and lung diseases when particulate pollution peaks."